Porous materials are utilized in the fields of foods, medical supplies, biotechnologies and the like, for instance, in the step of separating, filtrating, or condensing various gases, vapors and organic solvents. Recently, environmental issues become more serious on a global scale, and porous materials are also applied to purification devices of various environmental pollution gases.
In the separation of various gases and vapors, membrane separation is under examination for separating and refining condensed special gas component from multi-component gas mixture. An organic polymer film applied in the process of membrane separation is inappropriate for use under high temperature conditions on account that heat resistance is low. Further, the organic polymer film is easy to be corroded by fluid used, thereby it causes problems that lifetime and efficiency become lowered. In contrast, an inorganic membrane applied in the process of membrane separation is excellent in durability, heat resistance and thermal shock resistance. Accordingly, it is in the hope of being put into practice under the conditions that the organic polymer membrane is inapplicable, and its development is now underway.
In case of applying various kind of zeolite, sol-gel, CVD or other films having innumerable minuscule pores for gas separation, some degree of mechanical and thermal strengths are required, but efficiency is reduced an the thickness becomes large. Accordingly, as for a porous ceramic filter, asymmetric film structure and techniques for changing a functional film into thin and defectless one without deteriorating its function are necessary in order to restrain the deterioration of penetration efficiency and impart the strength to the thin membrane portion. In addition, there are many problems to be solved in a process for preparing a film of practically usable structure in terms of pressure resistance, reliability and sealing for use under severe conditions for example, at high temperatures.
Various studies have been made with regard to porous ceramic materials or in regard of processes for forming a ceramic membrane on a substrate, and various porous ceramic filters suitable for the above use are proposed. For example, JP-A-7-109116/1995 (The term “JP-A” used herein means “unexamined published Japanese patent application”) discloses a process for forming a fine crystalline zeolite film inside the pores and on the surface of an alumina substrate applying hydrothermal synthesis; JP-A-4-104973/1992 discloses a process for preparing porous ceramic materials using inorganic particles of a predetermined diameter range and a binder: JPA-2-175657 and JPA-2-175658/1990 disclose a process for preparing a ceramic composition comprising the step of sintering silicon nitride powder and polysilazane; and JP-A-8-059364/1996 discloses a process for preparing porous ceramic materials of a uniform pore size distribution by the use of ceramic powder and ceramic precursor.